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Marc
17 comments
aeva

@mdiluz Amazing. Really, this is perfectly logical if you think about it. It's safe to remove the while loop because it does not have any side effects, and it is also safe to remove the instructions following the while loop because the loop as written provably doesn't terminate.

Ness from earthbound standing in front of a skull in a cave. Text reads "No problem here", which is the message the game gives you if you try to check an object that doesn't have any specific information available.
9 Feb 2023 at 1:54 | Open on mastodon.gamedev.place
Bee O'Problem

@aeva @mdiluz but where does the call to unreachable come from? Your logic only explains the loop being optimized out unless I'm missing something.

9 Feb 2023 at 3:14 | Open on mastodon.gamedev.place
RAOF

@beeoproblem @aeva @mdiluz The reasoning I've seen is:
You generate code for main() and unreachable(). You use the UB to dead-code eliminate main(). Now the code for unreachable() is where the code for main() was; ie: it has become the entrypoint.

9 Feb 2023 at 3:32 | Open on toot.cat
aeva

@RAOF @beeoproblem @mdiluz if you're missing a main function other functions don't get promoted to main. I tested it out in godbolt earlier, what happens is the ret instruction in main gets optimized out, thus the program counter just advances to the next instruction in memory which is the other function. The entry points for main and the other function still exist.

9 Feb 2023 at 4:14 | Open on mastodon.gamedev.place
aeva

@RAOF @beeoproblem @mdiluz gcc and msvc both do what you'd expect. Clang and zig's c++ compiler both optimize away the loop and the return.

9 Feb 2023 at 4:15 | Open on mastodon.gamedev.place
Marqin

@beeoproblem @aeva @mdiluz the return call from main() got optimised away and so the CPU doesn’t jump away, and keeps taking further instructions from the assembly, which are the body of unreachable()

9 Feb 2023 at 3:45 | Open on mastodon.gamedev.place
DELETED

@aeva @mdiluz but why doesn't also remove the unused function? It's never called so even if the loop terminates, it should never fall through to the function right?

This breaks abstraction/encapsulation, as well as the security and control boundaries functions serve.

9 Feb 2023 at 5:20 | Open on mastodon.social
Zoe

@AgreeableLandscape @aeva @mdiluz because of linking, the compiler doesn't know if the object will be linked. The function can be called from another object.

9 Feb 2023 at 5:47 | Open on social.librem.one
aeva

@ekg @AgreeableLandscape @mdiluz this exactly. To Agreeable Landscape's concerns, if you mark "unreachable" as static (indicating it only exists for the translation unit), then clang removes it. It's not a big spooky zero day, this is just how linking works 🤷‍♀️

9 Feb 2023 at 10:32 | Open on mastodon.gamedev.place
Brion Vibber :blobcatcoffee:​
Jon A. Cruz

@brion @mdiluz oh, clang, you silly little thing. Ya start with extraneous warnings and now we see what's next

9 Feb 2023 at 2:23 | Open on mstdn.social
Þór Sigurðsson

@mdiluz apparently it doesn't work just everywhere
9 Feb 2023 at 6:52 | Open on mast.ttk.is
Alberto Ⓥ

@thor offtopic: this is zsh, right?

9 Feb 2023 at 7:05 | Open on mastodon.social
Louis Dureuil

@thor
Yep, Undefined Behaviour is unreliable. The compiler was written to produce a binary whose behaviour is as described under certain rules, and this program does not respect these rules. It is like using an item in anti-gravity. If it hasn't been specifically designed for this use case, chances are that it assumed gravity. If you're lucky it might work even in anti gravity but you wouldn't count on it. Now the question is if the rules of C++ make sense 🫣
@mdiluz

9 Feb 2023 at 7:21 | Open on mastodon.social
Marc

@thor yeah, it's mostly because of a specific rule in clang's optimiser

9 Feb 2023 at 9:02 | Open on mastodon.gamedev.place
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